1. Field of the Invention
The present invention relates to liquid crystal light valves of the type utilized in video projection systems and, in particular, to a reflective liquid crystal light valve which utilizes a hydrogenated amorphous silicon photodiode to activate the liquid crystal in the presence of a photoactivation signal.
2. Discussion of the Prior Art
In the operation of a liquid crystal light valve, the electro-optical properties of a liquid crystal are used to modulate a projection light based upon a photoactivating writing light. For the light valve to function properly, it must be capable of switching a drive voltage from a photoactivated layer to the liquid crystal layer in response to application of the writing light to the photoactived layer.
Liquid crystal light valves based on cadmium sulfide photoconductors are well known. For example, U.S. Pat. No. 3,824,002 entitled "Alternating Current Liquid Crystal Light Valve", issued to Beard on July 6, 1974 teaches a light valve wherein transparent indium-tin-oxide (ITO) electrodes are formed on the interfaces of two glass cover plates. The ITO electrodes are electrically connected to alternating current sources through associated leads. A silicon dioxide insulating film is formed on either side of a nematic liquid crystal to provide electrical and chemical isolation between the liquid crystal and the electrodes. Spacers are utilized to maintain a suitable gap between the insulating layers and to prevent the liquid crystal from escaping. Positioned sequentially on the side of the liquid crystal from which photoactivating writing light enters the light valve are a zinc sulfide dielectric mirror, a light blocking layer of cadmium telluride and the cadmium sulfide photoconductor.
The above-mentioned Beard patent teaches the basic principles of operation of an alternating current reflective liquid crystal light valve, i.e., the photoconductor must be impedance matched to the liquid crystal and the photocapacitance of the photoconductor must be altered in response to the writing light.
U.S. Pat. No. 3,976,361 entitled "Charge Storage Diode With Graded Defect Density Photocapacitive Layer" issued to Fraas et al. on Aug. 24, 1976, teaches the advantages of providing a reflective liquid crystal light valve of the type described in the above-mentioned Beard patent with a high sensitivity photoactive layer comprising a dual-layer cadmium sulfide photodiode wherein the first layer is relatively pure cadmium sulfide and the second layer, which forms the interface with the cadmium telluride light blocking layer, is cadmium sulfide film which has a higher defect center density in the form of selenium atoms.
While cadmium sulfide light valves of the type taught by Beard and Fraas et al. can provide bright, high resolution and high contrast projected images, they suffer from an unacceptable drawback for some applications. This drawback is "persistance", or a latent image in the projection which fades slowly dependant on "charge integration", that is, on the length of time that the writing light is applied to the cadmium sulfide. Persistance is due to the presence of electron traps deep within the 2.4 e.v. wide cadmium sulfide forbidden band.
In addition to the persistence problem described above, cadmium sulfide photoconductors suffer from the further disadvantage that, as a material, cadmium sulfide is difficult to process. Basically, it is difficult to deposit cadmium sulfide on a substrate. Beard describes the thermal deposition of cadmium sulfide on a heated substrate. Typically, this is accomplished by reactive sputter deposition. Cadmium sulfide sputter-deposited under these conditions of thermal stress results in a photoactive layer which has a tendency to peel from the substrate, rendering the light valve inoperative. Even if the cadmium sulfide layer can be successfully deposited, the deposition procedure results in a matte finished surface which then must be polished to permit successful operation of the light valve. Polishing cadmium sulfide to the required finish is known to be a difficult task. Furthermore, there is not a clear understanding either of the physics of cadmium sulfide or of the equivalent circuit of cadmium sulfide based light valves.
U.S. Pat. No. 4,032,954 entitled "Silicon Single Crystal Charge Storage Diode" issued to Grinberg et al. on June 28, 1977 discloses a liquid crystal light valve which utilizes a photoactive layer fabricated from single crystal silicon which is doped with a slow recombination center element, such as silver. The silver-doped single crystal silicon addresses the persistence problem by providing deep hole traps without the slow electron traps common to cadmium sulfide. However, because of the much greater thickness of the single crystal silicon, resolution is severely degraded unless compensating techniques are utilized. For example, Grinberg et al. utilize a photolithographic technique to produce a matrix of small p-n junction pixels formed within the crystalline silicon layer to provide charge localization.
The use of a single crystal silicon photoactive layer rather than cadmium sulfide provides several advantages. Larger area silicon crystals are more readily available than is true for cadmium sulfide. Furthermore, silicon in the single crystal form, as distinguished from the polysilicon form, provides better semiconductor qualities and more consistent quality control then does cadmium sulfide. Single crystal silicon also provides better quality lattice matching and its characteristics and processing are better understood than are those of cadmium sulfide.
While manufacturing techniques for single crystal silicon are better understood than those of cadmium sulfide, the crystalline silicon manufacturing process is still quite complex since a very intrinsic starting material is required for liquid crystal light valve applications. Furthermore, it is difficult to obtain crystalline silicon in large sizes. Also, both the cost of crystalline silicon and the special processing required to tailor it for use as a photoconductive element in a light valve make its use in this application quite expensive. Additionally, the many steps required to process crystalline silicon for use in a light valve greatly reduces yield. In devices of the type described by Gringerg et al, resolution is limited by the number of discrete pixels used.
L. Samuelson et al have reported on the use of amorphous silicon as a dc-coupled photoconductive resistive divider in a reflective liquid crystal light valve. Se "Fast photoconductor coupled liquid-crystal light valve", Appl. Phys. Lett. 34(7), 1 April 1979, pp. 450-452. Samuelson et al describe a light valve comprising a molybdenum electrode which is evaporated onto a glass substrate. A layer of boron-doped amorphous silicon is deposited on the electrode in a glow-discharge apparatus. A 12 micron thick Mylar spacer defines the liquid crystal cavity. A Sn-In-O electrode deposited on a glass substrate completes the device.
The primary focus of Samuelson et al in using amorphous silicon, however, was to develop a "slow" photoconductor, i.e. one which exhibited increased persistence by storing photogenerated charges in deep traps within the photoconductor structure. The Samuelson et al. light valve requires current bleed-off over time after short pulses of addressing laser light have been applied to the amorphous silicon. The amorphous silicon layer acts as a photoresistor with resistance increasing continuously after the light pulse, as the charge is carried through the device in the form of time-decreasing current. This is in marked contrast to devices operating on the entirely different principle of formation and modulation of a depletion layer, which makes use of continuous photogeneration of electron/hole pairs to maintain a given depletion depth and photocapacitance.